Communication booth



APll 2, 1946- R. L LEADBETTER 2,397,609

COMMUNICATION BOOTH Filed Feb. 10, 1945 Patented Apr. 2, 1946 UNITED STATES PATENT OFFICE COMIVIUNICATION BOOTH Application February 10, 1943, Serial No. 475,330

14 Claims.

This invention relates to improvements in communication booths of the type which is adapted for use in noisy surroundings, and which provides a quiet zone within the booth where communi cation can be carried on, over the telephone or between two persons standing in the booth. In particular, the invention relates to that type of communication booth in which the quiet zone of communication is obtained by means of a soundabsorbing construction applied to the interior surfaces of the walls and acoustically exposed to the space within the booth, whereby sound energy within the booth is absorbed. The invention is of especial value with that type of booth in which the interior is not completely enclosed by the walls, but in which the entrance, or other portion of the walls, is permanently open, whereby the space within the booth is in direct communication with the space outside the booth. A communication booth of this type is described in Weiss Patent 2,141,117, granted December 20, 1938. The invention may also be used with a booth in which the interior is totally enclosed by the walls and a door.

In the booths of this general type known heretofore the walls are of imperforate sheet material, and a thick layer of porous sound-absorbing material is arranged as a lining against the interior surfaces of the walls. If desired, a facing of foraminous sheet material may be arranged to cover the layer of sound-absorbing material and present 'an attractive surface for the interior of the booth. As is known, such a foraminous facing is substantially transparent to sound and does not affect appreciably the properties of the porous material for absorbing the sound energy which is present within the booth.

A communication booth of the type described, in general, has excellent quieting properties. However, it has been found that where the sound in the surrounding space includes substantial amounts in the low frequency range, such low frequency sound is present in the booth in a relatively high degree of intensity. With the continually widening use of communication booths, many applications have been encountered where the level of low frequency sound is relatively high. This is especially the case in mills and factories where heavy machineryA -is employed. It has been found that within narrow frequency bands the intensity of such low frequency sound is greater within the booth than in the surrounding space and is objectionable to the user. Atmospheric pressure Waves, or mechanical vibrations, having frequencies below the lower limit of the audible range, which, for the average person, is 20 cycles per second. are sometimes present and such .energy is also augmented in the booths known heretofore with the result that a throb-v bing sensation is sometimes experienced which is quite disturbing.

It is the object of the present invention to provide an improved communication booth of the character described which is free of the described objection and within which the low frequency sound, as well as the high frequency sound, is at such low intensity that it does not interfere with the reception and transmission of messages within the booth, or have a disturbing effect upon the user.

There is no sharp line of demarcation between the high frequency sound which is quieted effectively and the low frequency sound which is augmented in the booths known heretofore, but there is rather a gradual transition from one effect to the other. At 250 cycles per second there is a definite quieting and below cycles per second there is a definite augmentation of the sound within narrow bands of frequencies. The range varies somewhat with booths having different dimensions. The transition point, or cut-olf frequency, is between these two points, and, for purposes of definition, may be considered to be at approximately 200l cycles per second. Where high frequencies and low frequencies are referred to herein, frequencies above and below this cut-off, respectively, are contemplated, the latter `term including the frequencies below the audible range mentioned heretofore.

Briefly, the improved quietness in the low frequency range is obtained by making the exterior wall member of the booth foraminous and providing an air-impervious, flexible, non-vibratile, membrane adjacent the interior surface of the exterior wall member, that is, between the exterior wall member and the lining of sound-absorbing material.

In the drawing:

Fig. 1 is a perspective view of a communication booth made in accordance with the present invention; and

Fig. 2 is a fractional sectional view of a wall of the booth.

The booth is enclosed upon the rear lli, the two sides Il and the top I2. The front is permanently open, whereby the space within the booth is in direct communication with the space outside the booth. The walls are supported upon spaced-apart supports I3, thereby providing openings etween the supports, which openngs also provide direct communication between he space within the booth and that outside the booth. The booth is sufficiently high to accommodate a person standing up, and is usually deeper from front to back than it is wide because such an arrangement provides the best quieting results. a specic example, the inside dimensions of the booth were as follows: height 6 feet inches, depth 3 feet width 2 feet.

A shelf i5 may be provided at the rear of the booth and a telephone instrument it may be installed upon the shelf, the shelf being at a convenient height for a person using the telephone while standing. A lighting fixture l'l may be provided in the rearward portion of the top of the both.

En Fig. 2 is illustrated a transverse section of a wall which gives the booth the desired acoustic properties. Usually all walls of the booth are of this construction. rlhe wall consists of the perfoi-ated exterior` wall member 2o which is of stur construction and is composed of sheet material, such as sheet metal, wood, plywood, hard pressed fibrous material, phenol-formaldehyde resin, and the like. |lhe wall member 2t may be of vari-- ous thicknesses. Sheet metal as thin as 0.0i inch and plywood as thick as 3A inch may be used satisfactorily. Thin plywood, having a thickness of approximately Tse inch, is quite satisfactory. The wall member is provided with small openings 2l distributed over the area thereof, whereby the said wall member is foral., A thick layer of porous sound-absorbing material 22 is arranged interiorly of exterior wall member Eil, and a membrane is arranged between the wall member E@ and the layer 22. The layer m may be oi varying thickness, and in a er- H rspeciflc example was 3 inches thick. It may be composed of any of the well known sound-absorbing materials, such as porous mineral wool or a similar porous blanket of vegetable nbers, animal hair, or the like. Adjacent the interior surface or the sound-absorbing layer 22 is located a facing member 23 of thin foraminous sheet material, such as perforated metal, plywood, hard pressed fibrous material, phenol-formaldehyde resin, and the like. Facing member 23 is exposed to the interior of the booth. As has been stated heretofore, the facing 23 is substantially sound transparent, and from the standpoint of acoustics, the sound-absorbing material is acoustically exposed to the space within the booth in the same manner as if it were directly exposed to said space.

If desired, bracing members 2d may be arranged between wall member E?) and facing 23 to increase the rigidity of the wall structure. Members 2d may be in the form of strips of a width equal to the distance between wall member 2@ and facing and may be arranged in the form of a grid of intersecting strips. Bracing members of this character are especially desirable where the exterior wall member and facing are composed of thin, flexible sheet material, such as thin plywood. I

in accordance with the present invention, there is placed immediately adjacent wall member 2i?, that is, between wall member 2t and the sound-absorbing layer 22, a membrance 25 which is substantially air-impervious, flexible and nonvi-bratile. By the term non-vibratile, it is meant that the membrane is not capable of undergoing free vibration, either by being struck with a percussion instrument such as a hammer, or by the removed.

impingement thereon oi an atmospheric pressure Wave. With low frequency sound, the membrane undergoes the initial movement caused by the pressure wave and then comes to rest in much the same manner that a suspended blanket responds to the impact of a thrown baseball. That is, it undergoes forced vibration, as distinguished from free vibration in which the membrance continues to vibrate after the driving force has been The displacement ci the membrane is in phase with the pressure variations of the atmospheric pressure waves and as a result the low frequency sound is not reflected but is transmitted to the space on the opposite side of the membrane. There is a loss of energy in this transmission, due to the inertia of the membrane. As the frequency of the sound becomes higher, the membrane responds less and less, until with the high frequency sound there is no appreciable response. The membrane can, therefore, be said to be relatively transparent to low frequency sound, but substantially opaque to high frequency sound. There is no sharp cut-off frequency between the high and low frequency ranges, but there is rather a gradual diminishing of responsiveness as the frequency increases and the cutoif frequency is most accurately thought of as the frequency approximately mid-way between those at which the membrane reaches the substantial maximum of responsiveness and unresponsiveness, respectively. The membrane is selected so that its responsiveness diminshes over the range from to 250 cycles per second mentioned heretofore, and its cut-oil" frequency is between these two frequencies and for purposes or" deiinition may be stated to be at approximately 200 cycles per second. An example of a suitable membrane is one composed of felted fibers cemented together by asphalt, such as ordinary roofing felt. Other materials which may be used are linoleum, sheet cork impregnated with drying oils or gums whereby it is air-impervious, heavy oil cloth and the lik The membrane 25 is not rigidly secured to the wall member 2), but is mounted in such a manner that the vibrations which may be induced in one are not transmitted mechanically to the other. The consideration of primary importance is that the membrane 25 be adapted to respond to iinpinging low frequency sound substantially in .ependently of wall member 2l). For example, it may be supported adjacent the top thereof. if the membrane is subdivided into sections by the bracing members 2t, as is shown in Fig. 2, each section is supported adjacent the top thereof. The remainder of the membrane, or of the section', is in unattached relation to the wall memb'er 2li. A suitable mea-ns for supporting the membrane are staples 255 which are driven through the membrane and into the Wall inember 2li, as shown in 2. Other means may be employed, such as gluing the membrane to the wall member 2t in a line erin spots along the top of the membrane. The points of attachment need not be confined to the top, however, but may be distributed at spaced-apart locations throughout the area of the membrane, so long as they are located far enough apart that vlbra tions of the wall member 2! are not transmitted to the membrane. A relatively large panel, such as is present in a wall of the communication booth, may vibrate to a small extent as a single diaphragm. but most ofy its vib'ratory action is con,- tributed by relatively small sections thereof .which vibrate as independent diaphragms. By spacing the points of attachment relatively far apart, for example, 18 inches cr more, this vibratory action of the small sections of the wall member is not transmitted to the membrane. If the membrane is attached firmly to the wall member 20 throughout its area, it closes the small openings 2l of the wall member 20 and the two vibrate together as a unit and become, in effect, a single vibratory panel.

The openings 2i may be of any desired size or shape, but it is preferred that they be small and not exceed the approximate equivalent of circular openings having a diameter of 1A; inch. To avoid being covered over when the booth is decorated, they should not be smaller than 0.02 inch in diameter. Openings 35 inch in diameter` are quite satisfactory.

With exterior wall members of rse inch plywood and openings 33e inch in diameter and spaced approximately 3 inches apart between centers, a substantially lower level of low frequency sound is obtained over that present in a similar booth having imperforate exterior` wall members. The improved quietness is quite noticeable and renders the booth more satisfactory for use. In such arrangement the aggregate area of the openings is equal to approximately 0.075% of the area of the wall member. With exterior wall members of the same material and openings having a diamn eter of 3/4 inch, substantially the same low-frequency sound level is obtained when the spacing` between the openings is 3 inches. This conforms to an aggregate area of openings equal to approximately 5.0% of the area of the wall member. Experiments with openings having a diameter between inch and 1% inch show that a spacing of approximately 3 inches produces a closely similar level of low frequency sound.

In general, the openings must be within a definite proximity to each other to produce a lower level of low frequency sound, and this proximity is approximately the same for openings of different size. The areas of the imperforate sections between the openings is the determining factor and these must be kept small in order to obtain the desired results. If the imperforate sections are large, they exhibit properties similar to those of an imperforate wall member and, in

proportion to their size, detract from the benee iicial effect which is the object of the invention. To provide a lowered level of low frequency sound which is of denite value in the use of the boothy the average spacing between the centers of adjacent openings should not exceed approximately 3 inches.

The openings 2l should be distributed substantially uniformly over the area of the wall member, as is evident from the foregoing description. By this it is not meant that the size spacing of the openings must be quite uniform. Decorative and other non-uniform patterns may be employed as long as large imperi'orate areas are not thereby formed.

With exterior wall members of the same material and 133s inch diameter openings spaced approximately 1 inch apart, a level of low frequency sound is obtained which is appreciably lower than obtained with the wider spacing mentioned heretofore. The l inch spacing conforms to an aggregate area of openings equal to approximately 0.5% of the wall member area. The low frequency sound reaches the lowest level with the same size openings spaced approximately 3A inch apart, which conforms to an aggregate area of 75 openings equal to approximately 1.0% of the wall member area. An increase in the area of openings beyond this point does not affect the low frequency sound level, and the latter remains substantially the same. The results are not impaired by increasing the open area and the openings may occupy any desired area greater than the value of 0.075% of the wall member area mentioned heretofore. However, from the structural standpoint, the open area should not exceed approximately of the area of the wall member, and it is preferred that it does not exceed approximately 5% of the wall member area. In general, equivalent results are obtained if the open area is increased somewhat as the thickness of the wall member is increased.

The action by which the improved quietness at low frequencies is obtained is not fully understood, but it is believed to beas follows. An explanation will first be given of the action which takes place in the booth described in the Weiss Patent 2,141,- 117. The sound enters such booth through the open entrance and the spaces between the bottom supports. It also impinges on the exterior surfaces of the exterior wall members and sets them in vibration. Within the low frequency range, there are narrow bands of frequencies within which the exterior wall members respond very readily, and considerable sound of such frequency is transmitted to the interior of the booth by the diaphragmatic action'of the exterior wall members. The soundabsorbing construction comprising the thick layer of sound-absorbing material and the foraminous facing is effective in absorbing the high frequency sound, but not the low frequency sound. 'I'he high frequency sound, therefore, both that entering through the open entrance and that due to the vibration of the imperforate exterior wall members, is absorbed and quieted.

The low frequency sound entering through the open entrance passes through the sound-absorbing construction and impinges on the imperforate exterior wall members, where it is largely reflected and passes through the sound-absorbing construction again and into the space within the booth. The low frequency sound caused by the diaphragmatic action of the imperforate exterior wall members likewise passes through the sounclabsorbing construction and into the interior of the booth. As a result, the level of the low frequency sound energy is built up within the booth. It is a matter of experience that the intensity of low frequency sound is greater within the booth than it is in the surrounding space.

When the construction in accordance with the present invention is used, the sound enters by way of the open entrance and the spaces I4, and passes through the small openings of the foraminous wall member 20. Because of its foraminous condition, the wall member Eil does not transmit sound by diaphragmatic action. The high frequency sound entering by way of the open entrance and spaces i4 is absorbed by the porous material 22 in the same manner as in the known booth. That passing through the foraminous wall member 2D is stopped by membrane 25, which acts as a barrier to the passage of such sound into the booth. High frequency sound is, therefore, effectively quieted.

The low frequency sound entering by way of the open entrance and spaces I4 passes through the material 22 and impinges upon the membrane 25. The membrane responds to these vibrations and they are not reflected into the booth because the displacement of the membrane is in `phase with the pressure variations of the air pressure waves, as explained heretofore. vIt is transmitted by the membrane, in reduced intensity, and thence passes through the foraminous wall member 2U and to the space surrounding the booth. With respect to the low frequency sound impinging upon the exterior surf-aces of the booth Walls, such sound passes through the foraminous wall members 2B and is transmitted by the membrane, in diminished intensity, in the lsame way as explained heretofore with respect to the low frequency sound passing 'in the opposite direction. It then passes through the porous material 22 and into the interior of the booth. The intensity o-f the low frequency sound which is transmitted into the booth in this manner is insufficient to cause any interference or disturbance to the user of the booth.

If an imperforate wall member is employed in place of foraminous wall member 20, in the construction shown in Fig. 2, the beneficial effects of the invention are not realized. It is believed that in such construction the action is substantially the same as in the booth known heretofore. Since the membrane transmits low frequency sound, its presence in such construction does not prevent the building up of low frequency energy within the booth.

The invention is not limited to a communication booth having an open entrance or open spaces in the walls. It may be used with a totally enclosed booth having a door. In such case all of the sound which enters the booth fis transmitted by the Walls and the door. The action by which quieting is obtained is similar to that explaineL heretofore.

I claim:

1. A communication booth comprising a composite wall enclosing a zone of communication within .said booth, said composite wall comprising an exterior wall member having openings therein distributed substantially uniformly over the area thereof, a membrane arranged interiorly of said exterior wall member said membrane being so constructed and arranged that it is airimpervious, flexible and non-vibratile and is responsive to low frequency sound energy and substantially unresponsive to high frequency sound energy, the cut-off frequency between said high and low frequencies being between approximately 150 cycles per second and approximately 250 cycles per second, and a layer of porous soundabsorbing material arranged interiorly of said membrane and exposed acoustically to the interior of said booth.

2. A communication booth comprising a composite wall enclosing a zone of communication within said booth, said composite wall comprising an exterior wall member having a multiplicity of openings therein distributed substantially uniformly over the area thereof and occupying at least approximately 0.075% of said area, a membrane arranged interiorly of said wall member and supported in non-vibration-transmitting relation to said exterior Wall member, said membrane being so constructed that it is air-impervious, flexible and non-Vibratile and is responsive to low frequency sound energy and substantially unresponsive to high frequency sound energy, the

cut-off frequency between said high and low fre` quencies being between approximately 150 cycles per second and approximately 250 cycles per second, and a layer of porous sound-absorbing material arranged interiorly `of said membrane and exposed acoustically to the interior of said booth.

3. A communication booth comprising a composite wall enclosing a zone of communication within said booth, said composite wall comprising an exterior Wall member having a multiplicity of openings therein distributed over the area thereof and occupying at least approximately 0.075% of said area, the average spacing between the centers of adjacent openings not exceeding approximately 3 inches, a membrane arranged interiorly of said exterior wall member and supported in non-vibration-transmitting relation to said exterior wall member, said membrane being so constructed that i1; is air impervious, iiexible and non-vibratile and is responsive to low frequency sound energy and substantially unresponsive to high frequency sound energy, the cutoff frequency between said high and low frequencies being between approximately cycles per second and approximately 250 cycles per second, and a layer of porous soundeabsorbing material arranged interiorly of said membrane and exposed acoustically to the interior of said booth.

4. A communication booth comprising a composite wall enclosing a zone ofcommunication within said booth, said composite wall comprising a foram-inous exterior wall member, 'a membrane arranged interiorly of said foraminous wall member, said membrane being so constructed and arranged that it is air-impervious, flexible and nonvibratile and is responsive to low frequency sound energy and substantially unresponsive to high frequency sound energy, the cut-off frequency between said high and low frequencies being between approximately 150 cycles per second and approximately 250 cycles per second, and a layer of porous sound-.absorbing material arranged interiorly of said membrane and acoustically exposed to the space within said booth.

5. A communication booth comprising a composite wall enclosing a zone of communication within said booth, said composite wall comprising an exterior wall member pervious to sound having a frequency lower than approximately 250 cycles per second, a membrane arranged interiorly of said exterior wall member, said membrane being so constructed and arranged that it is air-impervious, iiexible and non-vibratile and is responsive to low frequency sound energy and substantially unresponsive to high frequency sound energy, the cut-olf frequency between said high and low frequencies being between approximately 150 cycles per second and approximately 250 cycles Iper second, and a layer of porous soundabsorbing material arranged interiorly of said membrane and acoustically exposed to the space within said booth.

6. A communication booth comprising a wall enclosing a Zone of communication within said booth, said wall comprising a, membrane which is so constructed and arranged that it is air impervious, flexible and non-vibratile and is responsive to low frequency sound energy and substantially unresponsive to high frequency sound energy, the cut-off frequency between said high and low frequencies being between approximately 150 cycles per second and approximately 25o cycles per second, and a layer of porous soundabsorbing material arranged interiorly of said membrane and acoustically exposed to the space within said booth.

7. A communication booth comprising a wall enclosing a zone of communication within said booth, the wall being omitted from a portion of the sides of said booth whereby the space within said booth is in direct communication with the space outside said booth, said wall comprising a membrane which is so constructed and arranged that it is air-impervious, flexible and non-vibratile and is responsive to low frequency sound energy and substantially unresponsive to high frequency sound energy, the cut-ofi frequency between said high and low frequencies being bctween approximately 150 cycles per second and approximately 250 cycles per second, and a layer of vporous sound-absorbing material arranged in teriorly of said membrane and acoustically exposed to the space within said booth.

8. A communication booth comprising a ccmposite wall enclosing a zone of communication within said booth, the wall being omitted from a portion of the sides of said booth whereby the space within said booth is in direct communication with the space outside said booth, said composite wall comprising an exterior wall member having a multiplicity of openings therein distributed substantially uniformly over the area thereof and occupying at least approximately 0.075% of said area, a membrane arranged interiorly of said exterior wall member, said membrane being so constructed and arranged that it is air-impervious, flexible and non-Vibratile and it is responsive to low frequency sound energy and sub- 1 stantially unresponsive to high frequency sound energy, the cut-oif frequency between said high and low frequencies being between approximately 150 cycles per second and approximately 250 cycles per second, and a layer of porous soundabsorbing material arranged interiorly of said membrane and exposed acoustically to the interior of said booth.

9. A communication booth comprising a con.- posite wall enclosing a Zone of communication within said booth, the wall being omitted from a portion of the sides of said booth whereby the space within said booth is in direct communication with the space outside said booth, said compcsite wall comprising an exterior wall member having a multiplicity of openings therein distributed over the area thereof and occupying at least approximately 0.075% of said area, the average spacing between the centers of adjacent openings not exceeding approximately 3 inches, a membrane arranged interiorly of said exterior wall member, said membrane being so constructed and arranged that it is air impervious, flexible and non-vibratile and is responsive to low frequency sound energy and substantially unresponsive to high frequency sound energy, the cutoff frequency between said high and low frequencies being between approximately 150 cycles per second and approximately 250 cycles per second, and a layer of porous sound-absorbing material arranged interiorly of said membrane and exposed acoustically to the interior of said booth.

l0. A communication booth comprising a composite wall enclosing a zone of communication within said booth, the wall being omitted from a portion of the sides of said booth whereby the space within said booth is in direct communication with the space outside said booth, said composite wall comprising an exterior wall member having a plurality of openings therein distributed over its area, the average spacing between the centers of adjacent openings not exceeding approximately 3 inches, a membrane arranged interiorly of said exterior wall member and supported in non-vibration-transmitting relation to said wall member, said membrane being so constructed that it is air-impervious, exible and non-vibratile and is responsive to low frequency sound energy and substantially unresponsive to high frequency sound energy, the cut-olf frequency between said high low frequencies being between approximately cycles per second and approximately 250 cycles per second, and a layer of porous sound-absorbing material arranged interiorly of said membrane and exposed acoustically to the interior of said booth.

ll. A communication booth comprising top, rear and side walls, and having the front thereof ermanently open, said walls each comprising an exterior wall member having a plurality of openings therein distributed over its area, the average spacing between the centers of adjacent openings not exceeding approximately 3 inches, a membrane arranged interiorly of said exterior wall member, said membrane being so constructed and arranged that it is air-impervious, iiexible and non-vibratile and is responsive to low frequency sound energy and substantially unresponsive to high frequency sound energy, the cutoff frequency between said high and low frequencies being between approximately 150 cycles per second and approximately 250 cycles per second, and a layer of porous sound-absorbing material arranged interiorly of said membrane and exposed acoustically to the interior of said booth.

12. A communication booth comprising substantially vertical walls and a top wall, said vertical walls being mounted upon spaced-apart supports, the spaces between said supports providing direct communication between the space within said booth and the space outside said booth, said walls each comprising an exterior wall member having a plurality of openings therein distributed over its area, the average distance between the centers of adjacent openings not exceeding approximately 3 inches, a meme brane arranged interiorly of said exterior wall member, said membrane being so constructed and arranged that it is air-impervious, flexible and non-vibratile and is responsive to low frequency sound energy and substantially unresponsive to high frequency sound energy, the cut-olf frequency between said high and low frequencies being between approximately 150 cycles per second and approximately 250 cycles per second, and a layer of porous sound-absorbing material arranged interiorly of said membrane and exposed'acoustically to the interior of said booth.

13. A communication booth comprising a composite wall enclosing a zone of communication upon at least three sides and the top thereof, said composite wall comprising an exterior wall member having a plurality of openings therein distributed over its area, the average spacing between the centers of adjacent openings not exceeding approximately 3 inches, a membrane arranged interiorly of said exterior wall member, said membrane being so constructed and arranged that it is air-impervious, flexible and non-vibratile and is responsive to low frequency sound energy and substantially unresponsive to high frequency sound energy, the cut-off frequency between said high and low frequencies being between approximately 150 cycles per second and aproximately 250 cycles per second, and a layer of porous sound-absorbing material arranged interiorly of said membrane and exposed acoustically to the interior of said booth.

14. A communication booth comprising ay cornposite Wall enclosing a zone of communication within said'booth,v said composite Wall comprising,r an exterior wall member and a layer of porous sound-absorbing material arranged interiorly of said exterior wall member and exposed acoustically to the interior of said booth, said exterior wall member being substantially transparent to 10W frequency sound and substantially opaque to high frequency sound, the cut-off frequency between said 10W frequency sound -and said high frequency sound being between approximately 150 cycles per second and approximately 250 cycles per second'.

RALPH L. LEADBETTER. 

